“We were much more of an insular society back then,” said William Frey, a demographer at the Brookings Institution, a Washington think tank.

As of early Sunday, there were 299,058,932 people in the United States, according to the Census Bureau’s population clock. The estimate is based on annual numbers for births, deaths and immigration, averaged throughout the year.

The 300 millionth person in the U.S. will likely be born — or cross the border — in October, though bureau officials are wary of committing to a particular month because of the subjective nature of the clock.